1. Field of the Invention
The present invention relates to a method of annealing a semiconductor film using a laser light (hereinafter referred to as laser annealing) and to a laser irradiation apparatus for conducting the method (apparatus including a laser and an optical system for guiding a laser light emitted from the laser to an object to be processed). Also, the present invention relates to a method of manufacturing a semiconductor device including said step of laser annealing. Note that the term semiconductor device mentioned here indicates semiconductor devices in general which can function by utilizing semiconductor characteristics, and includes electro-optical devices such as a liquid crystal display device, a light emitting device and an electronic device including the electro-optical device as a component.
2. Description of the Related Art
In recent years, there has been widely studied a technique of conducting laser annealing to a semiconductor film formed on an insulating substrate made of glass or the like to crystallize the film, thereby improving crystallinity. Silicon is often used for the semiconductor film. In this specification, a method of crystallizing a semiconductor film with a laser light to obtain a crystalline semiconductor film is referred to as laser crystallization.
A glass substrate has such advantages that: it is inexpensive and has a wealth of processability; and a large-area substrate can be easily manufactured from the glass substrate in comparison with a synthetic quartz glass substrate often used in the prior art. This is the reason why the above study is being made. Further, the reason a laser is used for crystallization from choice residues in that the glass substrate has a low melting point. The laser can impart high energy only to a semiconductor film without increasing temperature much in substrate. Further, crystallization can be performed with the laser in a short time. Thus, remarkably high throughput is provided with the laser in comparison with a heating means using an electrically-heated oven.
The crystalline semiconductor film formed by performing laser irradiation has high mobility, and thus is actively used for an active matrix liquid crystal display device, for example, which is manufactured by forming thin film transistors (TFTs) using the crystalline semiconductor film and forming TFTs for a pixel portion and for a driver circuit on, for example, a glass substrate.
A laser beam oscillated from an Ar laser and an excimer laser or the like is often used as the laser beams. A method of using the Ar laser to perform laser crystallization is disclosed in Japanese Patent Laid-Open No. 6-163401 and Japanese Patent Laid-Open No. 7-326769, etc. The excimer laser has advantages that the excimer laser can be highly outputted and irradiated repetitively at a high frequency. Further, laser beams emitted from the excimer laser has the advantages of having a high absorption coefficient with respect to silicon films, which is often used as a semiconductor film.
As to the laser irradiation, a method of the laser irradiation in which the laser beam is formed by an optical system such that it becomes a elliptical shape, a rectangular shape and a linear shape on an irradiation surface and the periphery thereof, and the laser beam is shifted (or relatively shifting an irradiation position of the laser beam with respect to the irradiation surface) is superior in mass productivity and is excellent in technology. The “linear shape” described here means not a “line” in the strict sense but a rectangle (or a prolate ellipsoid shape) having a high aspect ratio. For example, it indicates a shape having an aspect ratio of 10 or more (preferably, 100 to 10,000). In this specification, a laser beam shape (laser beam spot) on the irradiation surface, an ellipsoid shape is referred to as an ellipsoid beam; a rectangular shape, a rectangular shape beam; a linear shape, a linear beam. If there is no definition for the laser beam spot particularly, it means a cross-section by a perpendicular plane with respect to the optical axis.
At an edge of elliptical shape, rectangular, or linear laser light formed on an irradiation surface or in the vicinity thereof by an optical system, the energy density in central part reaches a peak while in the edge part thereof is attenuated gradually due to aberration of a lens or the like (FIGS. 9A and 9B). In such a laser light, in order to perform annealing on an object to be irradiated, a region with enough energy density is around ⅕ to ⅓ of the entire irradiation surface including the central part of the laser light and is extremely narrow. In this specification, a region at edges of the laser light where the energy density is insufficient to perform annealing on an object to be irradiated is called an attenuation region.
As the substrate area is increased and the laser power is raised, it is now possible to form a longer elliptical shape beam, linear beam and rectangular beam. Annealing with such laser light is more efficient. However, the energy density of laser light emitted from a laser is smaller at its edge than around the center. Therefore, if laser light is expanded by an optical system more than prior art, attenuation in the attenuation region is intensified.
In comparison with the central part of the laser light, the energy density in the attenuation regions is not enough, for this reason, an object to be irradiated cannot be annealed sufficiently by using the laser light having the attenuation regions.
For example, when a semiconductor film is an irradiation object, a region of the semiconductor film that is annealed by the attenuation region and a region of the semiconductor film that is annealed by the highly uniform region including the central part have different crystallinity. Therefore, if this semiconductor film is used to manufacture TFTs, the electric characteristic of a TFT formed from the region that is annealed by the attenuation region is inferior to other TFTs and causes fluctuation among the TFTs on the same substrate.